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First of all get a good Antivirus software and scan your PC thoroughly after UPDATING it. Updating the Virus software is extremely important.Secondly after doing the above step install a Registry cleaning software. I recommend Uniblue Registry booster or Regcure. Virus and unclean registry are number 1 reasons to PC slow down.Check your hard disk if it is near to full. Use the Windows built in disk cleaning software to clean unnecessary files. After that De-fragmenting the disk will also help a lot. To de-fragment a disk Go to accessories in the start menu and then go to system tools. Find disk de-fragmentation and run it the PC performance will dramatically enhance but Virus and registry cleaning is recommended to be done first.

depends on what computer you have. you might wanna check one of those reg edit cleaners (be sure not to take a virus though). check if you have enough hardrive space (should always be ~15gb on your main partition). check your startup items, and remove those you dont want to. Check your drivers, if they are up to date (videocard driver ie).
and what always helps @ me, is a complete reinstall of the computer.

You can do a bit of detective work and run MSCONFIG.EXE froma command prompt and go through that - particularly the boot, services and startup entries. The startup entries in particular - as from there you can select what is and what is started at startup - and a process of elimination can maybe help you track down the problem. 'Registry cleaners' may help - but often are more problematic themselves thn they are 'fixers'. You could edit the registry by hand, using regedit, but that is NOT recommended unless you really know what you are doing. Do note that if you change the start-up programs (i.e. change their enable state), you will get a warning at start-up that you are running a custom start-up using msconfig.
As a last resort, a fresh installation of Windows will fix things. Running a virus checker might also be a good idea.
Allan

make sure that the chords are securely connected to the correct slot, and that the jumper pins for them are not either broken off or bent flat. if that isnt the problem try running a registry fixer like regcure or something to clear the original settings. or try to reset your BIOS

Error 21is caused by missing System files or damaged registry stuctures.The only way i think you can repair it is to download Regcure and scan your PC.Once the Registry is repaired you can then uninstall Nortonshttp://simple-pc-help.com/support/downloadb.php